1. Field of the Invention
This invention relates to control devices for movable lenses and, more particularly, to control devices for zoom lenses.
2. Description of the Related Art
Zoom lenses having the focusing function and the function of compensating for the image shift during zooming performed by one and the same lens group (or compensator) have been proposed, for example, in Japanese Patent Publication No. Sho 52-15226 or Japanese Patent Publication No. Sho 56-47533. In such a zoom lens, unlike the zoom lens of the type in which the above-described two functions are performed by respective individual lens groups, it becomes possible to minimize the size of the lens system and to shorten the minimum object distance.
In the above Japanese Patent Publication No. Sho 52-15226, based on the position information of the variator lens group, a position to which the compensator lens group has to move is determined by a computing process.
In the above Japanese Patent Publication No. Sho 56-47533, on the other hand, the amount of defocusing resulting from the change of the image magnification is sensed by the automatic focus detector to determine the adjustment of the compensator lens group.
By the way, the inventions of the above-mentioned publications would have no problem if the aforesaid computing process were done in real time. But, practically it takes a good deal of time. Therefore, particularly when zooming, immediate response of the compensator to the actuation of the variator cannot be permitted to occur, thereby making it difficult to sustain the in-focus state.
On this account, Japanese Laid-Open Patent application No. Sho 62-296110 discloses a technique of compensating for this response delay of the automatic focus detector. According to that document, a plurality of zones defined by the positional relationship of the variator lens and the compensator lens are set, and a unit movement amount of the compensator lens for every one of these zones corresponding to a predetermined movement amount of the variator lens is memorized. Then, on the basis of position information of the variator lens and the compensator lens at the time when a predetermined movement of the variator lens has been detected, a unit movement amount of the compensator lens is obtained according to the zone to be set by this position information. Further, on the basis of the direction of zooming and the information of the blur detected by the automatic focus adjusting device, the above-described unit movement amount is corrected. According to this corrected movement amount, the above-described compensator lens is moved. Thus, the compensation is effected.
However, in the above-described Japanese Laid-Open Patent application No. Sho 62-296110, although in a case where whether the blurring of the image in the middle of the course of movement of the variator lens group by the predetermined movement amount, that is, at a time during zooming, is in near focus or in far focus is ascertained, and a relatively smooth approach to the in-focus state can be expected, it is in the case of a photographic situation where a long time is necessary to discriminate between the near focus and the far focus that a sufficient performance becomes difficult to achieve. Particularly when zooming from the wide-angle side to the telephoto side, the probability of occurrence of blurring from an intermediate time during zooming owing to the relation with the depth of field is considered to be, in principle, very frequent.
Meanwhile, FIG. 10(A) is a schematic view illustrating the lens position control method for the inner focus type zoom lens and an example of lens arrangement. 1' is a first lens group; 2' is a second lens group; 4A' is a front lens of a relay lens group; 4B' is a rear lens of the relay lens group; 12' is an image detecting means at a focal plane; 13' is a focusing control (AF) circuit for detecting the in-focus state and controlling focusing; and 14' is a drive means controlled by the AF circuit 13' to carry out the position determination and driving of the rear relay lens 4B'
FIG. 10(B) to FIG. 10(D) show an example of the automatic focus adjusting device. In FIG. 10(B), a whole picture plane 17' of a video camera contains an area 18' from which a signal is taken out to measure the object distance. An actual object to be photographed is assumed to have a contrast 19'. In FIG. 10(C), part (a) represents this contrast portion. Then, part (b) is a Y signal output, part (c) represents the differentiated value of the Y signal, (d) is its absolute value, and part (e) is a signal after a peak hold has been applied, where the height A indicates the degree of focus (hereinafter called the "blur evaluation value"). FIG. 10(D) is a graph with the abscissa taken in the lens position and the ordinate taken in the blur evaluation value A, showing that the in-focus state is realized in the position B of the peak.
Another improved method is proposed in Japanese Laid-Open Patent application No. Sho 62-284316, in which depending on the position information of the variator lens and a lens which is used both for the compensation and the focusing function, or the position information of the variator lens and the distance adjusting member (distance ring), a unit movement amount of that lens which is used both for the compensation and the focusing function corresponding to a predetermined amount of movement of the variator lens is memorized. Each time the variator lens moves by the predetermined amount, unit movement amount memorized.
By the way, if, in the publicly known zoom lens and lens position control method shown in FIG. 10(A), the accuracy and speed of the input signal from the image detecting means 12' to the AF circuit 13' are high, there is no possibility for the image formed on the focal plane to blur or distort. In actual practice, however, by the response delay due to the cyclic distance measurement, etc., the possibility of lowering the accuracy of control of the rear relay lens 4B' is very high. Therefore, there is a serious problem that a large blurring is liable to occur.
Also, in the aforesaid improved method, it becomes a prerequisite to detect when the variator lens has moved by the predetermined amount. To obtain the high precision accuracy of movement of the aforesaid lens which is used both for the compensation and the focusing function, therefore, the amount of movement of the variator lens must be made much finer. Further, the moving speed of that lens must be made faster. Otherwise, it would take a considerably long time to correct the blurring produced.
A method of discriminating whether the lens having the focusing function is in the near focus position or in the far focus position by moving that lens in directions to the near focus position, and the far focus position and detecting the resultant change of the video signal, is known in, for example, Japanese Laid-Open Patent application No. Sho 55-76309, Japanese Laid-Open Patent application No. Sho 59-133783 and others.